Could you please paste screenshot or write by hand your field definition (from admin panel)?

I suspect what may be wrong.

Please have a look at default Companies recordset fields in admin. If your commondata is not nested, then you shouldn't use colon, Probably you should write just Brand, not Brand::key.

Please compare your commondata array to CRM/Access (second level) or Companies_Groups (top level). If yours is like those two then you shouldn't use ::key.

Nested commondata arrays are separated by slash '/', underscore '_' does nothing, and this colon '::' is used for chained select. Notice that Countries::Country means, get value of field Country from current record and get commondata value from Countries/<Current record Country value>.